1. Field of the Invention
The present invention relates to an engine system and a vehicle including the same, and more particularly to an engine system including a valve driving device that drives an intake valve and a vehicle including the engine system.
2. Description of the Background Art
There have been conventionally known valve driving devices operable to drive an intake valve in an engine such as a mechanical valve driving device including a lift variable mechanism that changes the lift amount of an intake valve, and a hydraulic valve driving device operable to control the lift amount of an intake valve at an arbitrary level by oil pressure.
In a conventional mechanical valve driving device including a lift variable mechanism, when the lift amount of the intake valve changes, the valve timing changes accordingly. Therefore, in order to achieve the optimum lift amount and the optimum valve timing at the same time, it is necessary to provide a timing variable mechanism independently of the lift variable mechanism. This complicates the structure of the valve driving device.
In a conventional hydraulic valve driving device, the lift amount and valve timing of the intake valve can arbitrarily be controlled, so that the structure is not complicated.
In the hydraulic valve driving device, however, fluctuations in the lift amount of the intake valve are greater than those in the mechanical valve driving device. Therefore, in an extremely low load state with a very small lift amount such as at idle, the ratio of the fluctuation amount in the lift amount of the intake valve to the lift amount of the intake valve is large. This greatly changes the quantity of air taken into the cylinder through an opening where the intake valve is provided in the extremely low load state such as at idle. Consequently, the air-fuel ratio is liable to change, and the engine speed of the engine is likely to fluctuate accordingly.
An internal combustion engine (engine) that controls the power of the engine using a throttle valve at idle (in an extremely low load state) and in a high load state and by fully opening the throttle valve in a low to middle load state has been proposed (see for example JP 11-117777 A).
In the disclosure of JP 11-117777 A, the power of the engine is controlled using the throttle valve at idle (in the extremely low load state), and therefore when the lift amount of the intake valve fluctuates, fluctuation in the air-fuel ratio can be controlled, and fluctuation in the engine speed of the engine can be controlled accordingly.
In the disclosure of JP 11-117777 A, however, since the power of the engine is controlled using the throttle valve at idle (in the extremely low load state) and in the high load state, and the power of the engine is controlled using the intake valve in the low to middle load state, it is necessary to switch between the control by the throttle valve and the control by the intake valve among different load states of the engine.
Therefore, an additional driving device such as an actuator must be provided to fully open the throttle valve independently of the accelerate operation in the low to middle load state. This complicates the structure of the engine.